1. Field of the Invention
The present invention relates to a seat cushion pad composed of polyurethane foam, especially a seat cushion pad for use of a vehicle, especially a car, where a vibration is accompanied with the use of the seat.
2. Description of the Related Art
With respect to the comfortability of a car seat cushion, JASO B-407 regulates a standard on the vibration transmissibility characteristics. For attainment of comfortable feeling, it is effective, in general, to remarkably damp the vibration in a frequency region that makes persons feel uncomfortable. The frequency of vibration that makes persons feel uncomfortable is in the region of 4-10 Hz, and the vibration transmissibility at 6 Hz is conventionally employed for estimating the degree of the uncomfortable feeling described the regulation in the JASO. It is said that the vibration at the frequency about 6 Hz tends to cause carsickness, especially. Therefore, also in the development of the car seat cushion pad composed of polyurethane, much effort are devoted to suppress the vibration transmissibility in such frequency region.
Japanese patent publication H03-140319(A1) discloses a polyurethane foam which have a lower resonance frequency than usual, so as to remove the uncomfortable feeling of seat cushion.
The seat cushion pad according to the above prior art, which has suppressed vibration transmissibility characteristics at 6 Hz, shows large resonance amplitude (resonance peak) at the frequency region of 2 to 4 Hz. Therefore, when person sitting on a seat cushion using such polyurethane foam disclosed in the prior arts, often feels an instability of the body according to the vibration while travelling. Namely, such polyurethane foam can not protect sufficiently the disagreeable feeling that person""s body bounds up and down at a low frequency (feeling by shaking).
Accordingly, for the seat cushion pad for vehicle, a car for example, such characteristics listed below are required to prevent carsickness and fatigue of drivers and passengers.
i) suppressed vibration transmissibility characteristics at a frequency region about 6 Hz
ii) stability of the body of the person, who is sitting on the seat cushion and is driving the car
iii) a lower vibration transmissibility characteristics at a wider frequency region
However, according to a theory of a vibration engineering, in general, when the resonance amplitude in a vibration transmissibility characteristics at a frequency region of 2 to 4 Hz is lowered, the vibration transmissibility at a higher frequency (for example, 6 Hz), which is related to the agreeability of the seat cushion, increases. And the vibration transmissibility characteristics (vertical axis: vibration transmissibility; horizontal axis: frequency) shows broad curve as a whole. Namely, the lowering of the vibration transmissibility and the lowering of the resonance amplitude in a vibration transmissibility characteristics curve are contradictory requirements to each other.
Thus, the object of the present invention is to provide a seat cushion pad which shows lower resonance amplitude at 2-4 Hz and simultaneously shows the same or lower vibration transmissibility at a frequency region of about 6 Hz, compared to a conventional seat cushion pad. Thus, a seat cushion pad of the present invention shows a high damping characteristics.
The inventors of the present invention have studied eagerly the vibration transmissibility characteristics of a seat cushion pad made from urethane foam, and found that the aforementioned object can be achieved, when the hysteresis loss of a core layer of a urethane foam is less than 17% and the air permeability of the urethane foam including the skin portion falls in a special region. And the invention is accomplished.
A seat cushion pad according to the present invention is composed of polyurethane foam having a skin layer and a core layer, which is molded by supplying polyurethane foam raw material composition into a mould, characterized in that the seat surface has a single-layer structure, that the air permeability is between 0.05 and 1.5 cfm (cubic feet/min, i.e., ft3/min), and that the hysteresis loss of the core portion is 17% or below.
It is preferable that the air permeability of the seat cushion pad according to the present invention is between 0.1 and 1.5 cfm (2.81 to 42.2 L/min) and that the hysteresis loss is 15% or below. The air permeability shall be measured according to the regulation of ASTM D-1564 (DOW method).
The hysteresis loss of the core portion shall be measured using the sample which belongs evidently to the core layer, though the skin layer and the core layer is continuously formed and there is not any sharp boundary between them.
The seat cushion pad according to the present invention has a less resonance amplitude in the vibration transmissibility characteristics curve than that of a conventional seat cushion pad, and its vibration transmissibility at about 6 Hz is equal or less than a conventional seat cushion pad, as will be shown by evaluation results of the examples described later. Namely, according to the present invention, irrespective of the shape, it is possible to manufacturing a seat cushion pad, having a lower resonance amplitude and a lower vibration transmissibility at frequency of 6 Hz than conventional seat cushion pad.
The inventors of the present invention obtained new knowledge that the resonance amplitude in the vibration transmissibility characteristics is influenced by the air permeability: the lower the air permeability is, the lower the resonance amplitude is. And it is deduced that it is effective to lower the air permeability range, in such a manner as defined in the present invention, for lowering the resonance amplitude. Thus the range of the air permeability of the seat cushion pad according to the present invention is between 0.05 and 1.5 cfm (1.41 to 42.2 L/min). The lower limit of the air permeability range is defined with respect to the stability in the seat cushion pad forming. Namely, from a viewpoint of the stability of foaming, it is preferable that the air permeability is over 0.1 cfm, and particulary it is preferable that it is over 0.3 cfm (8.43 L/min). For lowering the resonance amplitude at 2 to 4 Hz, it is preferable that the air permeability is less than 1.5 cfm (42.2 L/min).
The measured value of the air permeability of polyurethane foam does not include the effect of the covering material of the seat cushion such as cloth, which will be applied to the seat cushion pad for producing the seat, the final product. However, the measured value of the air permeability includes the effect of the skin layer.
On the other hand, the hysteresis loss of the core layer, one of the physical properties defining the present invention, has almost no relation with the air permeability, and depends on the kind of the polymer which constitute the polyurethane foam. Further, the inventors obtained a new knowledge that the resonance frequency of a seat cushion pad composed of polyurethane foam depends on the hysteresis loss of the core layer, more precisely, the former is proportional to the later, and that the resonance frequency is inversely proportional to the resonance amplitude. Thus, it is deduced that an utilization of material having small hysteresis loss of core layer is effective in lowering the resonance frequency, in order to realize a seat cushion pad having a lower resonance amplitude and a lower vibration transmissibility at 6 Hz. It is preferable that the hysteresis loss of the core layer is 17% or below. And it is more preferable that the loss is 15% or below. The less the hysteresis loss of core layer is, the more preferable it is.
As explained above, for the first time, the object of the present invention is attained, by a seat cushion pad composed of polyurethane foam defined by the present invention, in which, on one hand, the air permeability as a whole of the foam including the skin layer, which is a physical properties to lower the resonance amplitude at a frequency of 2 to 4 Hz, is designed to fall in a predetermined range, and on the other hand, the hysteresis loss of the core layer, which is a physical properties to lower the vibration transmissibility in a frequency region of about 6 Hz and has no correlation with the air permeability, is designed to fall in a predetermined range. It is preferable to apply the seat cushion pad according to the present invention in making a pad having a thickness of 60-140 mm at its seating portion.
According to the present invention, it is preferable that the density of the polyurethane foam of the core portion is 50-65 kg/m3. In such a case, a seat cushion pad, having a air permeability between 0.05 and 1.5 cfm, a hysteresis loss of the core layer 17% or below and a as a compression modulus conforming to the requirements for a seat cushion pad for use of a car, can be securely obtained. And such density of the core portion is effective for lowering of the resonance amplitude in 2 to 4 Hz and the vibration transmissibility at about 6 Hz.
It is preferable that the aforementioned seat cushion pad is composed of polyurethane foam raw material composition, which comprises at least,
a) 30 to 80 parts by weight of polyether polyol having hydroxyl value of 20 to 40 mgKOH/g and having ethylene oxide unit at its terminal,
b) 70 to 20 parts by weight of polymer polyol having hydroxyl value of 15 to 30 mgKOH/g
(total parts by weight of the polyol (a+b) components is 100),
c) 0.1 to 2 parts by weight of silicone surfactant,
d) blowing agent, and
e) polyisocyanate compounds.
When the seat cushion pad is made from such raw materials having the aforementioned constitution, the freedom of the adjustment of the hysteresis loss is higher, hence, a seat cushion pad having a air permeability of 0.05-1.5 cfm and a hysteresis loss of 17% or below can securely obtained.
It is more preferable that the hydroxyl value of the polyether polyol(a), which has an ethylene oxide unit at its terminal and hence primary OH, is in the range of 24 to 36 mgKOH/g. In such a case, the air permeability and the hysteresis loss can be adjusted in a predetermined range, and also other advantageous characteristics as a seat cushion pad can be obtained.
The seat cushion pad, according to the present invention, can be characterized in the following points: the seat cushion pad is composed of polyurethane foam having a skin layer and core layer and having a one layer structure at its seating surface, and the polyurethane foam is molded by supplying (pouring or injection) the polyurethane foam raw material composition into a mould; the polyurethane foam material comprises:
a) 30 to 80 parts by weight of polyether polyol having hydroxyl value of 20 to 40 mgKOH/g and ethylene oxide unit at its terminal,
b) 70 to 20 parts by weight of polymer polyol having hydroxyl value of 15 to 30 mgKOH/g
(total parts by weight of the polyol components is 100),
c) blowing agent,
d) 0.1 to 2 parts by weight of silicone surfactant, and
e) polyisocyanate compounds;
wherein the silicone surfactant is selected so that the free foaming height of the free blow foam will be 225 mm or more.
According to the present invention, a seat cushion pad can be obtained, which has a low resonance amplitude in a vibration transmissibility characteristics curve, as well as a low vibration transmissibility at about 6 Hz, which is equivalent or less comparing to the level of a conventional seat cushion pad. Namely, irrespective of the shape of the seat cushion pad, it is possible to obtain a seat cushion pad having resonance amplitude as well as vibration transmissibility, either of which is lower than that of the conventional seat cushion pad.
The free foaming height of the free blow foam is an index representing the characteristics of a surfactant. Table 1 shows a model composition of the polyurethane foam material. The free foaming height of free blow polyurethane foam is measured by foaming the model composition material shown in the Table 1. The free foaming height of polyurethane foam is an index for estimating the characteristics of a surfactant, using the model polyurethane raw material composition.
The free foaming height of foam is obtained by measuring the height of free blow foam formed in a vessel having frustum cone shape, having a bottom diameter of 105 mm, a top diameter of 140 mm and a height of 195 mm. The total weight of the raw material composition of the foam is adjusted to be 147.3xc2x10.5 g.
In measuring the free foaming height, each of the polyol components including the polyols in the upper part of Table 1 are mixed homogeneously, then the temperature of both of the polyol components and the isocyanate compounds are adjusted to be 25xc2x0 C., and they are mixed homogeneously with each other, so as to form a foam under a predetermined condition (at room temperature). The mixing is carried out using a common mixer, which can mix the polyol compounds and the isocyanate compound to be homogeneous within about 5 seconds.
When the free foaming height is less than 225 mm, it is impossible to obtain a seat cushion pad having low resonance amplitude and low vibration transmissibility at 6 Hz, either of which is lower than those of common seat cushion pad. Also when the constituent of the polyurethane material are out of the range defined hereinbefore, it is impossible to obtain a seat cushion pad having a desired characteristics. The surfactant comprised of two or more compounds can be used, if the free foaming height evaluated using the mixture is over 225 mm.
It is preferable that the seat cushion pad has air permeability of 0.05-1.5 cfm (cubic feet/meter, i.e., ft3/min) (1.41 to 42.2 L/min), and that its hysteresis loss of the core layer is 17% or below.
It is more preferable that the seat cushion pad has air permeability of 0.1 to 1.5 cfm (2.81 to 42.2 L/min), and its hysteresis loss of the core layer is 15% or below.
When the air permeability and the hysteresis loss are adjusted to be within the aforementioned range, a seat cushion pad can be securely obtained, which has lower resonance amplitude in the vibration transmissibility characteristics curve and equal or lower vibration transmissibility at about 6 Hz than that of the conventional seat cushion pad.
It is preferable that the polyurethane foam raw material composition, for producing the seat cushion pad according to the present invention, further includes a low molecular weight crosslinking agent having hydroxyl value of 300 to 2000 mgKOH/g. It is more preferable that the hydroxyl value of the crosslinking agent is within the range of 400 to 1800 mgKOH/g. When the crosslinking agent is comprised of a plurality of compounds, the hydroxyl value is the mean value.
When such a low molecular weight crosslinking agent is used, the adjustment of the air permeability, the hysteresis loss of the core layer, and the compression modulus of a seat cushion pad can be facilitated. It is preferable that the amount of the crosslinking agent is 0.2 to 10 parts by weight, more preferably 0.5 to 6, to the 100 total parts by weight of the polyether polyol and the polymer polyol(polyol component).
It is preferable, according to the present invention, that the aforementioned silicone surfactant is polydimethylsiloxane or a compound represented by the chemical formula 1, where x is number of repeating units of the dimethylsiloxane, y is number of repeating units of the copolymer polyether substituted dimethylsiloxane, and the equivalent weight ratio x/y is 0.75/0.25xe2x89xa6x/y less than 1.00; and m,n is number of ethylene oxide unit and propylene oxide unit in the copolyether, respectively, and the equivalent ratio m/n is between 0.4/0.6 and 0.05/0.95. In the case of using another surfactant together, it is preferable that the aforementioned surfactant is contained over than 10 wt % of the total surfactant. 
In the chemical formula 1, X denotes organic residual group of carbon number 1 to 4. R denotes alkyl or acyl group.
The equivalent ratio x/y and m/n in the chemical formula 1 can be measured by NMR. In the measurement of these values, for example, the value of x/y can be obtained from the integrated peak strength of the H in Sixe2x80x94CH3 and the integrated peak strength of the H in the methylene group in the Sixe2x80x94CH2xe2x80x94, and the value of m/n can be obtained from the integrated peak strength of the H in the methylene group in the ethylene oxide unit and the integrated peak strength of the peak of the H of in the methyl group in the propylene oxide unit.
It is preferable that the aforementioned polyisocyanate compounds is a mixed diisocyanate, which is a mixture of toluene-diisocyanate (TDI) and diphenylmethane-diisocyanate (MDI), the equivalent weight ratio of them is in the region of 70/30 to 90/10.
When such polyurethane raw material is used, it is possible to produce securely a seat cushion pad, having the air permeability is in the region of 0.05 to 1.5 cfm and the hysteresis loss of the core layer is less than 17%.